Third parties will be offering Skype features by including the SkypeKit engine but only a few of those have hit the market and mostly in the form of Skype inside of hardware (like the Grandstream business desk phone and the LG internet television).
Updated: April 4, 2019
Latest issues added or updated
Server update
Without Skype for Business Server 2015 CU4 (build 9319.272 or later) or Lync Server 2013 CU8 HF1 (build 8309.973 or later), Skype for Business on Mac requires Mobility enablement on the server side. After updating to builds 9317.272 or 8309.973 or later, Mobility enablement will no longer be required.
You can download these updates at Updates for Skype for Business Server 2015 and Updates for Lync Server 2013.
You can also find more information at FAQ - Skype for Business on Mac.